1. Field of the Invention
The present invention relates generally to the field of emission control equipment for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants, etc.) and, in particular to a new and useful method and apparatus for reducing or preventing the poisoning and/or contamination of an SCR catalyst. In another embodiment, the method and apparatus of the present invention is designed to protect the SCR catalyst. In still another embodiment, the present invention relates to a method and apparatus for increasing the service life and/or catalytic activity of an SCR catalyst while simultaneously controlling various emissions. In yet another embodiment, the present invention relates to a method and apparatus for controlling, mitigating and/or reducing the amount of selenium contained in and/or emitted by one or more pieces of emission control equipment for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants, etc.). In still yet another embodiment, the present invention relates to method and apparatus for controlling the selenium speciation in one, or both, of a gas phase or an aqueous phase by the addition of at least one metal additive upstream of either a wet flue gas desulfurization (WFGD) unit and/or a dry flue gas desulfurization (DFGD) unit.
2. Description of the Related Art
NOx refers to the cumulative emissions of nitric oxide (NO), nitrogen dioxide (NO2) and trace quantities of other nitrogen oxide species generated during combustion. Combustion of any fossil fuel generates some level of NOx due to high temperatures and the availability of oxygen and nitrogen from both the air and fuel. NOx emissions may be controlled using low NOx combustion technology and post-combustion techniques. One such post-combustion technique involves selective catalytic reduction (SCR) systems in which a catalyst facilitates a chemical reaction between NOx and a reagent (usually ammonia) to produce molecular nitrogen and water vapor.
SCR technology is used worldwide to control NOx emissions from combustion sources. This technology has been used widely in Japan for NOx control from utility boilers since the late 1970's, in Germany since the late 1980's, and in the US since the 1990's. Industrial scale SCRs have been designed to operate principally in the temperature range of 500° F. to 900° F., but most often in the range of 550° F. to 750° F. SCRs are typically designed to meet a specified NOx reduction efficiency at a maximum allowable ammonia slip. Ammonia slip is the concentration, expressed in parts per million by volume, of unreacted ammonia exiting the SCR.
For additional details concerning NOx removal technologies used in the industrial and power generation industries, the reader is referred to Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright 2005, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., particularly Chapter 34—Nitrogen Oxides Control, the text of which is hereby incorporated by reference as though fully set forth herein.
Regulations issued by the EPA promise to increase the portion of utility boilers equipped with SCRs. SCRs are generally designed for a maximum efficiency of about 90 percent. This limit is not set by any theoretical limits on the capability of SCRs to achieve higher levels of NOx destruction. Rather, it is a practical limit set to prevent excessive levels of ammonia slip. This problem is explained as follows.
In an SCR, ammonia reacts with NOx according to one or more of the following stoichiometric reactions (a) to (d):4NO+4NH3+O2→4N2+6H2O  (a)12NO2+12NH3→12N2+18H2O+3O2  (b)2NO2+4NH3+O2→3N2+6H2O  (c)NO+NO2+2NH3→2N2+3H2O  (d).
The above catalysis reactions occur using a suitable catalyst. Suitable catalysts are discussed in, for example, U.S. Pat. Nos. 5,540,897; 5,567,394; and 5,585,081 to Chu et al., all of which are hereby incorporated by reference as though fully set forth herein. Catalyst formulations generally fall into one of three categories: base metal, zeolite and precious metal.
Base metal catalysts use titanium oxide with small amounts of vanadium, molybdenum, tungsten or a combination of several other active chemical agents. The base metal catalysts are selective and operate in the specified temperature range. The major drawback of the base metal catalyst is its potential to oxidize SO2 to SO3; the degree of oxidation varies based on catalyst chemical formulation. The quantities of SO3 which are formed can react with the ammonia carryover to form various ammonium-sulfate salts.
Zeolite catalysts are aluminosilicate materials which function similarly to base metal catalysts. One potential advantage of zeolite catalysts is their higher operating temperature of about 970° F. (521° C.). These catalysts can also oxidize SO2 to SO3 and must be carefully matched to the flue gas conditions.
Precious metal catalysts are generally manufactured from platinum and rhodium. Precious metal catalysts also require careful consideration of flue gas constituents and operating temperatures. While effective in reducing NOx, these catalysts can also act as oxidizing catalysts, converting CO to CO2 under proper temperature conditions. However, SO2 oxidation to SO3 and high material costs often make precious metal catalysts less attractive.
As is known to those of skill in the art, various SCR catalysts undergo poisoning when they become contaminated by various compounds including, but not limited to, certain phosphorus compounds such as phosphorus oxide (PO) or phosphorus pentoxide (P2O5). Additionally, it is also well known that SCR catalysts degrade over time and have to be replaced periodically at a significant cost and loss of generating capacity. In a typical 100 MWe coal plant the downtime and cost associated with the replacement of underperforming catalyst can be in the neighborhood of one million US dollars or more.
More particularly, as the SCR catalysts are exposed to the dust laden flue gas there are numerous mechanisms including blinding, masking and poisoning that deactivates the catalyst and causes a decrease in the catalyst's performance over time. The most common catalyst poison encountered when burning eastern domestic coal (i.e., coal mined in the eastern United States) is arsenic. The most common catalyst poison encountered when burning western domestic coal (i.e., coal mined in the western United States) is phosphorus and calcium sulfate is the most common masking mechanism. One method of recycling the used catalyst is the process called regeneration washing or rejuvenation. The initial steps of the regeneration process involve the removal of these toxic chemicals by processing the catalysts through various chemical baths in which the poisons are soluble. While this treatment process does an excellent job of removing the desired poisons it produces wastewater with very high arsenic concentrations.
In another situation, Powder River Basin/Lignite coal plants, any coal/biomass co-combustion, or any coal/bone meal co-combustion or even pure biomass combustion power plants will suffer from phosphorus contamination of their SCR catalysts.
Additionally, beyond controlling NOx emissions, other emission controls must be considered and/or met in order to comply with various state, EPA and/or Clean Air Act regulations. Some other emission controls which need to be considered for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants, etc.) include, but are not limited to, mercury, SOx, and certain particulates.
Furthermore, in most situations, if not all, it is desirable to remove various SOx compounds by way of either one or more wet flue gas desulfurization (WFGD) units or one or more dry flue gas desulfurization (DFGD) units from a flue gas. As is known to those of skill in the art, in conjunction with SOx removal it is common (and now required in most instances) to also remove and/or reduce the amount of mercury in a flue gas. One suitable method of mercury control is mercury oxidation and capture via the use of one or more halogen compounds to accomplish the aforesaid mercury oxidation and the subsequently capturing the oxidized mercury compound (e.g., in the form of a mercuric halide). It has been found that when mercury control is accomplished in whole, or in part, through the use of one or more halogen compounds (e.g., halide salts such as calcium bromide, etc.) that such compounds negatively impact on the selenium speciation in the flue gas which in turn negatively impacts the amount of selenium that is emitted via the liquid effluent outflow from one or more WFGD units, and or the particulate matter produced by one or more DFGD units that are utilized to control SOx in the same flue gas stream. However, it should be noted that the present invention is not limited to just the aforementioned situation. In fact, in one embodiment the present invention relates to a method and apparatus for controlling, mitigating and/or reducing the amount of selenium contained in and/or emitted by one or more pieces of emission control equipment for boilers, heaters, kilns, or other flue gas-, or combustion gas-, generating devices (e.g., those located at power plants, processing plants, etc.). In another embodiment, the present invention relates to method and apparatus for controlling the selenium speciation in one, or both, of a gas phase or a aqueous phase by the addition of at least one metal additive at any point upstream (as will be detailed below) of either a wet flue gas desulfurization (WFGD) unit and/or a dry flue gas desulfurization (DFGD) unit. In still another embodiment, present invention offers a method and apparatus by which to simultaneously control at least selenium speciation in one, or both, of a gas phase or an aqueous phase while further controlling at least one of gas phase phosphorus, gas phase sodium, gas phase potassium, and/or mercury in at least one emission from a combustion process.
Given the above, a need exists for a method that provides for any economical and environmentally suitable method and/or system to control selenium emission from one or more pieces of emission control equipment that are used in conjunction with a combustion process. Additionally, or alternatively, a need exists for a method to control selenium emission while simultaneously increase catalytic life span and/or catalytic activity of an SCR catalyst via the control of one or more gas phase compounds such as phosphorus, sodium, and/or potassium, and even in some instances the further ability to control mercury emission.